Currently, catalysts having a honeycomb structure (hereinafter referred to as honeycomb catalyst structures) are in use in order to purify the exhaust gases emitted from engines, etc. These honeycomb catalyst structures have a construction, as shown in FIG. 6, of partition walls 4 defining cells 3 and a catalyst layer 15 carried on the surface of partition walls 4. As shown in FIGS. 4 and 5, when an exhaust gas is purified using a honeycomb catalyst structure 60 (a honeycomb structure 11), the exhaust gas is allowed to flow into each cell 3 of the honeycomb catalyst structure 60 from its one end 2a, is allowed to contact with the catalyst layer (not shown) present on the surface of partition walls 4, and is discharged outside from the other end 2b of the honeycomb catalyst structure 60, whereby the exhaust gas is purified (see, for example, Patent Literature 1).
In purifying an exhaust gas using such a honeycomb catalyst structure, it is necessary to hasten the arrival of the to-be-purified components of the exhaust gas at the catalyst layer present on the surface of partition walls and increase the purification efficiency for exhaust gas. In order to increase the purification efficiency for exhaust gas, it is necessary, for example, to make small the hydraulic diameter of cell and make large the surface area of partition wall. In an example of the specific method therefor, the number of cells per unit area, that is, cell density is increased.
Here, it is known that the ratio of arrival (arrival ratio) of the to-be-purified components of exhaust gas at the catalyst layer on the surface of partition walls increases in inverse proportion to the square of the hydraulic diameter of cell. Therefore, a larger cell density results in an increased arrival ratio of to-be-purified components. However, pressure loss tends to increase as well in inverse proportion to the square of the hydraulic diameter of cell. Thus, there is a problem that pressure loss increases together with the increase in the arrival ratio of to-be-purified components.
Incidentally, the thickness of the catalyst layer on the surface of partition walls is ordinarily about several tens of μm. When the velocity of diffusion of to-be-purified components in catalyst layer is insufficient, the purification efficiency of honeycomb catalyst structure tends to be low. This tendency is striking particularly at low temperatures. Hence, in order to obtain a high purification efficiency for exhaust gas, it is necessary not only to increase the surface area of catalyst layer but also to reduce the thickness of catalyst layer to increase the velocity of diffusion of to-be-purified components in catalyst layer. Therefore, the increase in cell density produces an advantage of an increase in the surface area of catalyst layer but incurs a problem of an increase in pressure loss.
In order to reduce pressure loss with attainment of a high purification efficiency for exhaust gas, it is necessary to increase the inlet diameter of honeycomb catalyst structure and further reduce the velocity of the exhaust gas to be passed through the honeycomb catalyst structure. However, when the honeycomb catalyst structure has been made, for example, in a large size so as to be mounted on a vehicle or the like, the mounting thereof may be difficult because the space for mounting is limited.
Patent Literature 1: JP-A-2003-33664